Control device with current protected solid state relays

ABSTRACT

A control device, such as a smart thermostat, employs solid state relays as switches to activate and deactivate systems controlled by the device. By measuring the current flow through the power buses to one or more of the solid state relays of the control device, potentially damaging over current conditions can be distinguished from permissible transient over-current conditions and the control device can deactivate any solid state relays which would be damaged while allowing solid state relays which are experiencing allowable transients to remain operating. In the case of a severe over current condition, a current monitoring device can issue a fault signal, triggering an interrupt condition which will cause a processor in the controller to shut down the affected solid state relays very quickly.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a Continuation Application of U.S. patentapplication Ser. No. 15/878,977, entitled “Control Device with BulkHeating Compensation”, filed Jan. 24, 2018, which is a ContinuationApplication of, and claims priority to, U.S. patent application Ser. No.15/292,648, entitled “Control Device with Current Protected Solid StateRelays”, filed Oct. 13, 2018, all of which are incorporated by referenceherein for all purposes.

FIELD OF THE INVENTION

The present invention relates to a control device. More specifically,the present invention relates to a control device, such as anenvironmental control device for HVAC systems or the like, where thecontrol device employs solid state relays to activate and deactivatesub-systems controlled by the device.

BACKGROUND OF THE INVENTION

Control devices exist for many, many systems. A common control devicethat most people are familiar with is an environmental control devicesuch as an HVAC thermostat which can control of variety of environmentalfactors (heating, cooling, humidity, ventilation, etc.) in their homesor at other locations. While thermostats have been used for many years,such control devices have until relatively recently been simpleanalog/mechanical devices employing sensors such as bimetallic strips,etc. combined with mercury tilt switches which directly activate, ordeactivate, relays, contactors or other HVAC control devices.

Smart thermostats have now been developed which employ digitalprocessors executing potentially complex software programs to bettercontrol environmental factors. These smart thermostats are typicallyequipped with a variety of sensors (solid state temperature and humiditysensors, etc.) and other information (from remote sensors providingoccupancy information and/or remote temperatures, etc. and/or fromnetwork-connected servers providing weather conditions and forecasts,etc) which provide inputs to the executing software to control therespective HVAC systems. Such smart thermostats are becomingincreasingly popular, both because they typically allow remote controland monitoring (typically via Internet applications) of the operation ofthe smart thermostat and conditions within the controlled environment,but also because they provide increased user comfort and/or reduce HVACsystem energy usage. Examples of such smart thermostats include theEcobee₃ controller manufactured by Ecobee, 250 University Avenue, Suite400 Toronto, ON, Canada, M5H 3E5.

While such smart thermostats are significant improvements over priorthermostats, their design, manufacture and operation pose some uniquechallenges. For example, HVAC control systems typically require theswitching of significant amounts of electrical power to activate airconditioning compressors, circulating fans, etc. While prior artanalog/mechanical thermostats could typically handle significant levelsof electric power, including various exceptional conditions (faults), incontrast digital control devices are much more susceptible to spikes,over voltages, etc. and yet are expected to provide reliable service formany trouble free years. Further, while the advantages of smartthermostats are obvious, consumer behaviors still require that a smartthermostat be affordable, reasonably small in size and, above all,reliable. Meeting all of these criteria is a difficult task.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a novel controldevice which obviates or mitigates at least one disadvantage of theprior art.

According to a first aspect of the present invention, there is provideda control device for controlling one or more electrical subsystems inaccordance with a control program, comprising: a housing; a memorystoring an operating program; at least one environmental sensor withinthe housing to determine the temperature within the housing; at leastone solid state relay operable, when activated, to connect at least oneelectrical subsystem to a supply of power and further operable, whendeactivated, to disconnect the at least one electrical subsystem fromthe supply of power; at least one current measuring device to determinethe amount of current flowing from the supply of power to the at leastone electrical subsystem when the at least one solid state relay isactivated; and a processor operable to execute the operating program to:determine the amount of bulk heating produced by the determined amountof current and an associated predetermined electrical resistance; sumthe determined amount of bulk heating with the amount of bulk heatingproduced in the housing by other components to obtain a determined sumof the bulk heating in the housing; compensate the determinedtemperature value by an amount corresponding to the determined sum ofthe bulk heating in the housing to obtain a compensated determinedtemperature value; and to activate and deactivate the at least one solidstate relay to activate and deactivate the at least one electricalsubsystem as needed to maintain the compensated determined temperaturevalue within a preselected range of desired temperature values.

According to another aspect of the present invention, there is provideda method of controlling, with a control device, a HVAC system comprisingat least two subsystems: measuring, with a temperature sensor locatedwith a housing of the control device, a temperature; determining the sumof the bulk heating created by components and power flows within thehousing, comprising the steps of: (a) measuring the current flowingthrough at least one solid state relay to at least one of the at leasttwo HVAC subsystems; (b) determining a bulk heating value correspondingto the measured current flow; (c) determining a bulk heating valuecorresponding to bulk heating created by other components within thehousing; (d) summing all determined bulk heating values to obtain atotal bulk heating value; (e) compensating the measured temperature tocorrect for the determined total bulk heating value to obtain acompensated temperature value; comparing the compensated temperaturevalue to a preselected range of temperature values; and activating anddeactivating the at least one solid state relay to activate anddeactivate the at least one HVAC subsystem as needed to maintain thecompensated temperature value within the preselected range oftemperature values.

According to another aspect of the present invention, there is provideda method of protecting at least one solid state relay connected betweena load and a power bus within a control device from potentially damagingover current conditions, comprising the steps of: (a) determining a setof at least three safety thresholds, each threshold comprising a maximumcurrent flow level and a maximum permitted time therefore, thethresholds being defined according to the safe power dissipationcapacity of the solid state relay; (b) measuring a current flow throughthe solid state relay; (c) comparing that measured current flow to eachof the at least three safety thresholds; (d) determining if the measuredcurrent flow exceeds the maximum current flow level of at least one ofthe at least three safety thresholds; (e) if the measured current flowexceed the maximum current flow level of the at least one of the atleast three safety thresholds, determining if the current flow hasoccurred for a period exceeding the maximum permitted time defined forthat threshold and shutting down each of the at least one solid staterelays if the maximum permitted time threshold has been exceeded; (f)repeating step (e) for each of the at least three safety thresholds; and(g) repeating steps (b) through (f).

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the present invention will now be described, byway of example only, with reference to the attached Figures, wherein:

FIG. 1 is a schematic representation of a control device for an HVACsystem;

FIG. 2 is a block diagram of components of the control device of FIG. 1;

FIG. 3 is block diagram of a relay assembly of the control device ofFIG. 1;

FIG. 4 is a schematic representation of current flows through outputs ofthe control device of FIG. 1; and

FIG. 5 is a schematic representation of the power supply, shut downcircuitry and FETs of a solid state relay used in control device 20.

DETAILED DESCRIPTION OF THE INVENTION

A control device, in accordance with the present invention, is indicatedgenerally at 20 in FIG. 1. Control device 20 comprises a housing 24 witha front face 28 that includes at least a portion 32 which is transparentand through which a touchscreen 36 can be viewed and interacted with.Front face 28 can also include a motion sensor 40, which can be used asan occupancy sensor, detecting a user's presence and/or proximity tocontrol device 20. A printed circuit board (PCB) is located withinhousing 24 and much or all of the hardware of control device 20 ismounted on, or to, PCB which is discussed in more detail below.

Touchscreen 36 can display a wide variety of information, includingoperating messages, icons, controls and menus. For example, touchscreen36 can display an icon 44 indicting a current operating mode (such asheating or cooling); measured values such as a humidity level 48 and atemperature 52; user interface elements such as an input slider 56 topermit the input of a desired parameter, such as temperature, or adesired range for the parameter; a message window 60, where relevantmessages can be displayed to a user; and other user interface controlicons 64, 68 and 72 which can call up additional menus (64), informationsuch as outdoor weather (68) and additional settings menus (72).

As will be apparent to those of skill in the art, in addition to theexamples described above, touchscreen 36 can be configured and employedin a wide variety of manners to provide relevant information to a userand/or to allow a user to interact with control device 20 in a widevariety of ways.

FIG. 2 shows a block diagram of the hardware of control device 20, muchor all of which can be mounted on a PCB 42. Control device 20 includesat least one processor 100, which can be a microcontroller, amicroprocessor or any other suitable device as will occur to those ofskill in the art, non-volatile RAM 104 and volatile RAM 108. As will beapparent to those of skill in the art, neither, either or both of RAM104 and RAM 108 can be integral with processor 100, or can be separatediscrete devices or components, as desired.

Typically, non-volatile RAM 104 will store one or more programs forexecution by processor 100, as well as various parameters relating tothe execution of the programs and volatile RAM 108 will store data andworking values required by the programs.

Touchscreen 36 is operatively connected to processor 100, as is motionsensor 40, and control device 20 further includes a real time clock,either as a service provided in processor 100, or as a separatecomponent, not shown.

Control device 20 also includes at least one environmental sensor 112,which at a minimum is a temperature sensor but can also include otherenvironmental sensors, such as a humidity sensor, which determinerespective environmental conditions to be controlled and/or monitored.Typically, environmental sensors 112 in control device 20 will includeat least both a temperature sensor and a humidity sensor.

A wireless communication module 116 is operatively connected to anantenna 120 and to processor 100 to allow processor 100 to communicatewith communication networks such as the Internet and/or with additionalexternal sensors (not shown) via at least one wireless communicationprotocol, such as WiFi; Bluetooth; ZigBee; ZWave; Cellular Data, etc.

It is specifically contemplated that wireless communication module 116will allow at least one remote sensor, and preferably more than one, todetermine and report the temperature and/or humidity at other locationswithin the controlled premises at which control device 20 is installedand, also preferably, the temperature of the environment external tothose controlled premises.

Wireless communication module 116 also allows control device 20 tocommunicate with Internet based services (such as weather servers,remote monitoring systems, data logging servers, etc.) and withapplications used remotely by users of control device 20 to monitor andcontrol the controlled premises' environmental state.

As should now be apparent to those of skill in the art, control device20 operates to execute its programming and to monitor a variety ofenvironmental factors and other conditions and data and to compare thosefactors and conditions to a set of desired values, or ranges of values,which typically have been specified by a user of control device 20. Whenthe monitored values vary from the desired values by more than aselected amount, control device 20 will operate the appropriate subsystem(s) of the HVAC equipment of the premises to alter the respectiveenvironmental factor(s) to be closer to the desired value.

For example, if the temperature in the controlled premises is less thana user selected target temperature, control device 20 can activate afurnace (or other heating system) to raise the temperature and thendeactivate that HVAC equipment when the target temperature is achieved.If the temperature in the controlled premises is higher than the targettemperature, control device 20 can activate an air conditioner system(or a ventilation system, etc.) to lower the temperature, etc.

Control device 20 therefore further includes a relay assembly 140 toprovide appropriate control signals to HVAC subsystems, or any otherdevices controlled by control device 20 and relay assembly 140 isoperatively connected to processor 100 by a signal bus 122 such thatprocessor 100 can change the state of the outputs (described in moredetail below) of relay assembly 140. Relay assembly 140 is employed incontrol device 20 as at least some of the outputs at relay assembly 140require greater levels of power, and/or different voltages, than theoutputs of processor 100 can directly provide.

While for the purposes of illustration and clarity herein relay assembly140 is shown in the Figures as single subsystem, it should be understoodthat relay assembly 140 is not so limited and, in fact, the functions ofrelay assembly 140 can be implemented via appropriate components mountedat various locations on PCB 42 and connected, as needed, by appropriatePCB traces.

HVAC systems, if control device 20 is used to control the operation ofsuch systems, typically include a variety of (often six or more) controlsignal lines and power supply lines. With respect to power supply lines,some HVAC systems have a single transformer outputting 24 VAC for bothheating and cooling operations, while others have a separate heatingtransformer and cooling transformer, each of which output 24 VAC. In thelatter case, a power line “Rh” is the “hot” side of the heating-relatedtransformer of the HVAC system which connects to control device 20 and apowerline “Rc” is the “hot” side of the cooling-related transformerwhich connects to control device 20, as shown in FIG. 3.

In the former case of an HVAC system having a single transformer, in thepresent invention the hot side of that transformer can be internallyjumpered, by optional jumper 142, to provide both the Rc and Rh powerlines within control device 20. In many circumstances, either the Rc orRh power supply lines can also be used to power control device 20, or aseparate power supply can be employed, if desired. In FIG. 3, the Rcline serves to provide power to the rest of control device 20.

A common line (not shown) is connected between the other side(s) of thetransformer (or transformers, if two or more are present) and controldevice 20 to complete the power supply circuit.

Typical control signal lines from control device 20 can include a “G”line, for controlling an air circulating fan, an “O/B” line forcontrolling a heat pump reversing valve; a “Y” line (or Y1 and Y2 lines)for controlling an A/C or heat pump compressor, a “W” line (or W1 and W2lines) for controlling a furnace or auxiliary heater for a heat pump,and an “ACC” line for controlling accessories, such as dehumidifiers,humidifiers, ventilators, etc. In FIG. 2, five output terminals (G, O/B,Y, W, ACC) are shown on relay assembly 140, but different terminalsand/or greater or fewer numbers of output terminals can be provided asdesired.

While relay assembly 140 could employ electromechanical relays toenergize, and de-energize, the respective terminals this is notpreferred for a variety of reasons including: operating noise; powerconsumption and heat generation; reliability; size; and expense.Instead, in the illustrated embodiment relay assembly 140 employs solidstate relays, such as FET-based relays, to provide, or remove, controlsignals to the terminals in response to control signals from processor100.

One of the known factors that can affect the operation and accuracy ofcontrol device 20 is the self-heat produced by operation of controldevice 20. For example, processor 100, wireless communication module116, the backlight of touchscreen 36 (which preferably is equipped witha backlighting system), and relay assembly 140 all produce waste heat(typically referred to as bulk heating) as an inevitable byproduct oftheir operation. This bulk heating occurs within housing 24 ofcontroller 20 and thus environmental sensor 112 (which is also locatedwithin or on housing 24), will typically report a higher temperaturethan the ambient temperature in the environment outside housing 24, thusresulting in poor, or incorrect, temperature control by control device20.

Accordingly, it is known to apply a compensation factor to thetemperatures measured by internal sensors of control devices to removethe effects of bulk heating on those temperature measurements. One suchcompensation system is described in U.S. Pat. No. 9,016,593 toMetselaar, assigned to the assignee of the current invention.

In the Metselaar patent, compensation is performed by dynamicallymeasuring both the current and voltage supplied to the Metselaar controldevice by its power supply to determine a nominal value for the powersupplied to the control device and the controlled hardware (contactors,etc.). This determined nominal power supplied value is then used toselect a corresponding temperature offset value from a predeterminedtable of such temperature offset values. These offset values have beenpredetermined to relate the power supplied to the control device withthe bulk heating expected to correspondingly occur within the controldevice. The selected offset value is then subtracted from thetemperature value obtained from a temperature sensor within the controldevice housing to compensate the measured temperature value for the bulkheating within the control device.

While these prior techniques, and particularly the Metselaar technique,have improved the operating accuracy of smart thermostats, they stillproduce a less accurate result than may otherwise be desired. Forexample, the actual power consumption of the controlled subsystems/hardware can vary significantly between installations thusaffecting the underlying assumptions (and hence the accuracy) used tocreate the predetermined offset values.

Further, the differences in the actual bulk heating of a control devicewhen operating in different modes and/or with different active andinactive outputs can vary significantly and thus merely measuring powersupplied to the control device cannot provide for bulk heatingtemperature compensation with a desired level of accuracy.

In particular, as is well known to those of skill in the art, manydevices such as a solid state relay (such as a FET-based relay) producebulk heat proportional to the square of the current flowing throughthem. Thus, while values of bulk heat produced by the operation of somecomponents of a control device (e.g.—touchscreen backlight) will changelinearly and can be predetermined and stored for use in calculating acompensation factor, the amount of bulk heat produced within solid staterelays by their components and the resistance of printed circuit boardtraces cannot easily be accurately predicted as it will change withrespect to square (i.e.—non linearly) of the current flowing througheach solid state relay and the associated circuit board traces, which isin turn dependent upon the load connected to the terminals controlled bythe respective relays.

As mentioned above, in HVAC systems, typically the load connected toterminals of a control device can vary significantly between differentinstallations. For example, in one HVAC system the load (a contactor)connected to the W1 terminal may draw 3.5 amps while in another HVACsystem the load connected to the W1 terminal may only draw 1.5 amps. Aswill be apparent, the bulk heat produced by the solid state relay andcircuit traces handling the 3.5 amp load is over five times greater thanthe bulk heat produced by the same solid state relay and circuit traceswhen handling the 1.5 amp load.

Thus, it has proven to be difficult to provide bulk heating temperaturecompensation of a desired level of accuracy in prior art controldevices, such as smart thermostats and various assumptions and/oraverages have been employed in making such compensatory calculations inthe past for such systems, albeit with less than the best results.

In contrast, the present inventors have determined that by measuring theactual current being provided to relevant terminals of relay assembly140 a variety of advantages can be achieved, including obtaining a moreaccurate determination of the bulk heat produced in control device 20,allowing for a more accurate bulk heating compensation of temperaturesmeasured by environmental sensors 112.

Returning now to FIG. 3, relay assembly 140 is shown in more detail.Relay assembly 140 is connected to one or more ports of processor 100via signal bus 122 and can have a variety of output terminals forconnection to relevant HVAC devices. In the Figure, only the W1, W2, Yand G terminals have been shown for clarity, but it will be apparent tothose of skill in the art that a variety of other terminals can beincluded as desired. Relay assembly 140 also includes at least one of anRh and Rc power supply terminal, or equivalent, which supplies power(i.e. Vcc) to relay assembly 140 and thus to control device 20.

Each output terminal (W1, W2, Y, G) of relay assembly 140 is controlledby a respective switch 144, 148, 152 and 156, which in the presentembodiment are FET-based solid state relays, that are controlled byprocessor 100. Each switch 144, 148, 152 and 156 has a respectiveactivation signal (W1 On, W2 On, Y On and G On) which is part of signalbus 122.

Vcc is provided to the above-mentioned switches 144, 148, 152 and 156via one of two power busses, specifically an Rh bus 160 and an Rc bus164. Power supplied to Rh bus 160 flows through a current monitoringdevice 168 and, similarly, power supplied to Rc bus 164 flows through acurrent monitoring device 172.

While two power busses, each with a corresponding current monitoringdevice, are shown in FIG. 3, the present invention is not so limited andit is contemplated that in alternative implementations relay assembly140 can include a single power bus and associated current monitoringdevice, or can employ three or more power busses with correspondingassociated current monitoring devices, including an implementation witha separate power bus and associated current monitoring device for eachoutput terminal, if desired.

It is also contemplated that relay assembly 140 can include, in additionto a power bus with an associated current monitoring device, one or morepower busses which do not include associated current monitoring devicesfor cases wherein the power requirements for the outputs powered bythose power busses are fixed, or otherwise known, and thus the currentsupplied to those outputs is predetermined or otherwise known.

Returning again to FIG. 3, as mentioned above, Rh bus 160 is connectedto the Rh power supply terminal by current monitoring device 168, whichcan be any suitable device for monitoring current as will occur to thoseof skill in the art. In the illustrated embodiment, current monitoringdevice 168 provides an output voltage signal, RhCurrent, to processor100 via signal bus 122 and the RhCurrent voltage signal is proportionalto the current flowing through current monitoring device 168 to Rh powerbus 160 and switches 144 and 148 connected thereto. Current monitoringdevice 168 also preferably, but not essentially, provides a fault logicsignal RhFault (discussed below) to processor 100 via signal bus 122.

Similarly, Rc bus 164 is connected to the Rc terminal by currentmonitoring device 172, which is similar to current monitoring device168, which provides an output voltage signal, RcCurrent, to processor100 via signal bus 122 and the RcCurrent voltage signal is proportionalto the current flowing through it to the Rc bus and switches 152 and 156connected thereto. Current monitoring device 172 also preferably, butnot essentially, provides a fault logic signal RcFault (discussed below)to microprocessor 100 via signal bus 122. In the illustrated embodiment,Rc also provides operating power (Vcc) to the rest of control device 20and this power is supplied before current measuring device 172.

In operation processor 100 can, for example, cause the W1 terminal to beconnected to the Rh terminal, and hence energized via Rh bus 160, byasserting the “W1 On” signal line, closing switch 144. Similarly,asserting the “G On” signal line will close switch 156, causing the Gterminal to be connected to Rc bus 164.

By monitoring the currents measured by current monitoring devices 168and 172, processor 100 can calculate the amount of current flowingthrough the respective power bus 160 and 164 to switches 144, 148, 152and 156, etc. and can calculate a value for the bulk heat produced bythe current flowing through the respective switches and correspondingPCB traces.

In a simple case, processor 100 can have predefined total resistancevalues associated with each switch 144, 148, 152 and 156, eachpredefined total resistance value being a sum of the respective PCBcircuit trace resistances and the resistances across the respectiveswitches.

In such a case, processor 100 first determines the amount of the totalcurrent flow, reported by the respective current monitoring device,which flows through each activated switch (as described further below).When the respective current flow through each activated switch isdetermined, processor 100 can calculate the bulk heating produced withinrelay assembly 140 by, for each activated switch, determining theproduct of the square of the current flow allocated to the activatedswitch times the predefined total resistance value for the respectiveactivated switch and summing those determined values.

Once the bulk heating produced within relay assembly 140 is determined,this value is summed with the bulk heating produced in the remainder ofcontrol device 20 to obtain a determined sum of the bulk heating in thehousing. This latter value is determined by determining the voltage andcurrent provided to the rest of control device 20 (these values aredetermined by other current and voltage sensors, not shown) and theproduct of those determined values represents the bulk heating producedin control device 20, other than that produced in relay assembly 140.The determined sum of the bulk heating in the housing value is then usedto select a compensation factor to be applied to the temperaturemeasured and reported by environmental sensor 112 to obtain acompensated temperature value which represents the temperature of theenvironment surrounding control device 20 and which is used by controldevice 20 when it is employed to manage an environmental temperatureand/or relative humidity.

Determining the allocation of the measured current flows through eachbus to respective switches activated on the buses is achieved asfollows. FIG. 4 shows a representation of the ideal current flow throughbus 160, which in this example just includes two switches, 144 and 148.As can be seen, the total current flowing, I_(RhBus) is the sum of thecurrent (if any) flowing through switch 144 (i.e.—I_(W1)) and thecurrent (if any) flowing through switch 148 (i.e.—I_(W2)). Thus, thebulk heating produced by bus 160 is, in an ideal case, equal to thesquare of the current through switch 144 (I_(W1) ²) times the totalresistance of switch 144 and its associated PCB traces (collectivelyR_(W1)) plus the square of the current through switch 148 (I_(W2) ²)times the total resistance of switch 148 and its associated PCB traces(collectively R_(W2)). In this ideal case, if I_(RhBus) is measured whenonly a single switch is activated, then the current flow through thatswitch is equal to I_(RhBus) and can be measured and stored forsubsequent use to determine the appropriate allocation of measuredcurrent I_(RhBus) between respective switches when more than one switchis activated.

Thus, for example, if I_(RhBus) is measured at 2 amps when only switch144 is activated, current I_(W1) is deemed to be 2 amps(I_(W1)=I_(RhBus)) and processor 100 will store this value for futureuse. When switch 148 is subsequently also activated and currentI_(RhBus) is measured by current monitoring device 168 at 3.5 amps,processor 100 retrieves the stored value for current I_(W1) (i.e.—2amps) and subtracts the stored value from the measured value todetermine current I_(W2) to be 1.5 amps and the above-described bulkheating calculations are then performed with these values.

However many, if not most, HVAC systems employ inexpensive transformerswhich have significant internal resistances (i.e.—copper losses, etc.)and their supplied voltages decrease as the power drawn from thetransformer increases. Thus, to obtain a more accurate determination ofthe current flowing through each switch, in a preferred embodiment ofthe present invention this variability in the power supply voltage isalso considered when determining current flows through each activatedswitch.

For example, the nominal value of the supply voltage (VRhBus) on theRhBus is 24 VAC. When switch 144 is activated and current flows throughit to a load, I_(W1) may be measured at 2 A and VRhBus may be measured(by a voltage sensor, not shown) at 23.5 VAC and these values are storedby processor 100. Preferably, this is a dynamic process and is repeatedat regular intervals to quickly identify any changes in the operatingconditions of the HVAC sub systems and/or the operation of controldevice 20. In a present embodiment of the invention, this process isrepeated every 5 seconds.

If switch 148 is also activated and current flows through it to a load,IRhBus may now be measured at 3.5 A and VRhBus may now be measured at 22VAC. From these values, and the previously determined values of VRhBusand I_(W1) stored by processor 100, processor 100 can estimate that thenew value of I_(W1) under these conditions is

${2\mspace{14mu} A \times \frac{22\mspace{14mu} V}{23.5\mspace{14mu} V}} = {1.87\mspace{14mu} A}$and thus I_(W2) is 3.5 A−1.87 A=1.62 A at VRhBus=22V and processor 100stores these new values.

As will be apparent to those of skill in the art, if another switch X(not shown) is activated on the RhBus and IRhBus is measured at 5 A andVRhBus is measured at 20V, then, since processor 100 has stored valuesfor of 1.87 A for I_(W1) and 1.62 A for I_(W2), both at VRhBus=22V,processor 100 can determine that

${I_{W\; 1} = {{1.87\mspace{14mu} A \times \frac{20\mspace{14mu} V}{22\mspace{14mu} V}} = {1.7\mspace{14mu} A}}},{I_{W\; 2} = {{1.62\mspace{14mu} A \times \frac{20\mspace{14mu} V}{22\mspace{14mu} V}} = {1.47\mspace{14mu} A}}}$and I_(X)=5 A−1.7 A−1.47 A=1.83 A at VRhBus=20V and processor 100 willstore these values too.

A similar calculation is preformed when a switch is deactivated. In theexample above, when switch 148 is deactivated, we knew from the previousdetermination that I_(W2) was 1.47 A and VRhBus may now be measured at23V. Thus, the system proceeds to scale the remaining current flows anddetermines that

$I_{W\; 1} = {{1.7\mspace{14mu} A \times \frac{23\mspace{14mu} V}{20\mspace{14mu} V}} = {{1.955\mspace{14mu} A\mspace{14mu}{and}\mspace{14mu} I_{X}} = {{1.83\mspace{14mu} A \times \frac{23\mspace{14mu} V}{20\mspace{14mu} V}} = {2.1045\mspace{14mu}{A.}}}}}$

As will be apparent to those of skill in the art, for a variety ofreasons (i.e.—rounding errors, drift, etc.) the determined currentvalues may not precise total the measured currents reported by currentmonitoring devices 168 and 172. Therefore, each time that currentmonitoring devices 168 and 172 report a measured current to processor100, the difference between the total of the determined currents and themeasured current on each power bus is added, proportionally, to thedetermined current through each respective switch. In other words, ifswitches 144 and 148 are active on RhBus 160 and I_(W1) has beendetermined to be 1 A and I_(W2) has been determined to be 2 A and yetthe current reported by current monitoring device 168 is 3.1 A, thenprocessor 100 adds

$\frac{2}{3.1} \times 0.1\mspace{14mu} A$to the determined value for I_(W2) to get I_(W2)=2.0645 A and adds

$\frac{1}{3.1} \times 0.1\mspace{14mu} A$to the determined value for I_(W1) to get I_(W1)=1.03225 A.

This adjustment of the determined currents is preferably performed atregular intervals (in a current implementation of controller 20, every 5seconds) and each time a switch has been activated or deactivated on arespective bus.

As should now be apparent, by determining current flows through thesolid state switches of relay assembly 140 and calculating the bulkheating produced within housing 24 thereby and combining that value withthe determined value of the bulk heating produced elsewhere withinhousing 24, control device 20 can compensate temperature measurementsmade by environmental sensor 116 with a high degree of accuracy.

Another advantage of the present invention is the ability to provideimproved over-current protection for the switches in relay assembly 140.Specifically, switches such as solid state relays 144, 148, 152 and 156can be damaged by over-current events and solid state relays, such asthose implemented with FETs, can be fatally damaged by even briefover-current events. Accordingly, processor 100 can monitor I_(RhBus)and I_(RcBus) and can deactivate corresponding switches in the case of arespective measured current exceeding a preselected maximum permittedcurrent value. This can reduce the chance that switches 144, 148 152 and156 would be damaged by events such as an electrical short circuitduring an incorrect installation, or some other exceptional circumstancewhich leads to a temporary over current situation.

One of the challenges in implementing such an over current protectionscheme is the fact that control device 20 is expected to function whenconnected to a wide variety of HVAC devices and sub systems. Often, suchHVAC devices are controlled by contactors or other control devices whichare inductive loads on the output terminals of relay assembly 140. As iswell known, inductive loads often produce voltage spikes as theirinduced magnetic fields collapse when power is removed. It is notuncommon for such voltage spikes to reach, or exceed, 100 VAC in HVACsystems operating normally at a nominal 24 VAC.

Accordingly, control device 20 needs to be able to reliably distinguishbetween an unexpected fault condition which can result in damage to aswitch and/or connected equipment and a normal operating condition(transient) over-current event which can be tolerated. Upon detecting afault condition, control device 20 will then deactivate an affectedswitch sufficiently quickly to prevent damage being inflicted upon theswitch and connected equipment.

Desirably, any protection system should operate to: deactivate a switchexperiencing an over-current condition before any damage occurs to theswitch or connected equipment; deactivate a switch if the steady statecurrent through the switch exceeds a predefined threshold; allow aswitch experiencing a transient overload condition that will not damagethe switch or connected equipment to remain active; and allow a switchexperiencing repeated, but sufficiently infrequent, overload currenttransients to remain active.

As damage to FETs occurs when the FETs must dissipate too much powerover too long a period of time, raising the operating temperature of theFET junctions to a level at which the junction is damaged, both theamount of power dissipated and the time the FET is dissipating thatpower must both be considered.

The present inventors have determined that it is not practical toaddress all of the desired protection scenarios with a single fixedthreshold for detecting over current events. Instead, the presentinventors have established a set of thresholds that are triggered asdescribed below.

First, the present inventors examine the specific characteristics of theFETs employed in switches 144, 148, 152 and 156 to determine the maximumpower levels that the FETs can dissipate, in a given time, withoutcausing the FET to exceed the operating temperature at which they willbe damaged. Such information is typically available from the data sheetsprovided by the manufacturer of the FET or can be determinedempirically, etc.

In control device 20, the current flowing through each bus, asdetermined by respective current monitoring devices 168 and 172 arereported to processor 100 at regular intervals and in a presentimplementation these are reported every 1 ms. It is assumed that, if thecurrent through a respective bus exceeds a predefined threshold, asdescribed below, all activated switches on that bus should bedeactivated to ensure that no damage occurs to the switches or attachedequipment. As will be apparent to those of skill in the art, this is aconservative protection strategy as it is possible that, in a case wheretwo or more switches are activated on a bus determined to be in an overcurrent condition, only one switch might be supplying the excess currentand thus be susceptible to damage, but the present inventors haveadopted the conservative strategy of deactivating all active switches ona bus as this requires less processing and thus can be implemented morequickly. As will be apparent, if additional computing resources areavailable or if expected failure modes are much different, a strategy ofdeactivating only the switch supplying the excess current can beemployed instead.

In a specific example, it was determined that the FETs in the switches(144, 148, 152, 156, etc) at an expected nominal operating temperature,could tolerate a peak current of: 15.6 amps for ½ an AC sine wave(cycle); 13.36 amps for a full AC sine wave (cycle); 12.2 amps for twofull AC sine waves, 11.2 A for four full AC sine waves; and 9.96 ampsfor eight full AC sine waves, based on a 60 Hz AC cycle time.

From this information, the inventors determined a five layer set ofsafety thresholds for a maximum measured bus current (I_(peak)):

(1) if I_(peak) for a bus exceeds 12.5 amps then the respective busswitches are deactivated immediately;

(2) if I_(peak) for a bus is less than 12.5 amps but greater than 12amps for one full AC cycle, then the respective bus switches aredeactivated;

(3) if I_(peak) for a bus is less than 12 amps but greater than 11 ampsfor more than two full AC cycles, then the the respective bus switchesare deactivated;

(4) if I_(peak) for a bus is less than 11 amps but more than 10 amps formore than four full AC cycles, then the respective bus switches aredeactivated;

(5) if I_(peak) for a bus is less than 10 amps but greater than 3.5 ampsfor more than seven complete AC cycles, then the respective bus switchesare deactivated; and

(6) I_(peak) for a bus between 0 amps and 3.5 amps is considered thenominal operating condition.

In the case where a lower safety threshold value is exceeded for lessthan the maximum permitted number of cycles before I_(peak) increases toa higher safety threshold value, the count of the number of cyclesduring which the lower safety threshold value was exceeded will be addedto the count of cycles experienced at the higher safety threshold. Forexample, if I_(peak) is 5 amps for two cycles (of the seven permitted bythreshold (5)) and then I_(peak) increases to 10.5 amps, threshold (4)is considered to be triggered if I_(peak) remains over 10 amps for justtwo additional cycles (for the total of four AC cycles defined forthreshold (4)).

Further, anytime a safety threshold value is exceeded, but a safetythreshold is not triggered, i.e.—I_(peak) is 10.5 amps for one cycle,I_(peak) must go below the nominal value (i.e.—3.5 amps in this example)for a predefined number of AC cycles (in the current implementation atotal of 7 cycles) to allow the semiconductor junctions in the FETs tocool down to safer levels.

As will be apparent, the actual values utilized in the safety thresholdswill differ based upon the specific FETs employed in switches 144, 148,152 and 156, the expected ambient temperature at which control device 20will operate, the AC frequency (60 Hz vs 50 Hz), etc. and it is withinthe normal ambit of those skilled in the art of semiconductor devicecircuit design to be able to determine the maximum power dissipationlevels for a specific set of FETs and/or a specific set of expectedoperating conditions to produce suitable values for the set of safetythresholds.

Further, depending upon the expected operating conditions, it iscontemplated that a fewer or greater number of safety thresholds can bedefined within the set as desired.

In operation, processor 100 monitors the current flowing through eachbus (RhBus 160 and RcBus 168) and will compare the monitored maximumcurrent values (and the number of AC cycles that they have been at alevel of interest) to the predefined set of safety thresholds. When thecurrent through a switch exceeds a safety threshold, processor 100 willdeactivate the activated switches on the respective switch to preventthe FETs in the switches on that bus from becoming over heated and thusdamaged.

As an additional safeguard, each of current monitoring devices 172 and176 preferably have, respectively, a logic output Rc_(fault) andRh_(fault) which are active when the current passing through therespective current monitoring device is sufficiently high to saturatethe sensor in the device. In a preferred embodiment, processor 100treats Rc_(fault) and/or Rh_(fault) as an interrupt which is served byprocessor 100 and the interrupt service routine can immediately begin todeactivate all the switches in relay assembly 140 which are connected tothe respective Rh bus 160 or Rc bus 164 whose corresponding currentmonitoring device (160, 164 reported the fault. After the respectiveswitches have been deactivated, processor 100 can attempt to reactivatethe switches in a safe manner.

By handling Rc_(fault) and Rh_(fault) as an interrupt, rather than as ananalog input signal that must be digitized by an A/D converter (which isprovided as a component of processor 100, or as a separate component)processor 100 can shut down the relevant switches much more quickly thanwould be the case if a sample and hold cycle of an A/D converter had tobe performed. Thus, in the case of a severe over current event, shutdown can be achieved at a very quick rate.

When it is desired to deactivate a switch due to a fault or otherundesired condition, it is desired that the shutdown of the switch occurquickly. As is well known, when the gate voltage to a FET is removed, toshut down the FET, the FET changes from a saturated mode of operation,through a linear mode of operation and then to off. Due to theresistance characteristics of FETs, a significant amount of power isdissipated while they are in the linear mode and it is thus importantthat, in an over current condition, the FET is transitioned through thelinear mode region as quickly as possible.

FIG. 5 shows a schematic representation of switch 144 of control device20. As shown, switch 144 comprises two FETs 200 and 204 which areconnected between VRhBus and a load. In controller 20, the voltage usedto power processor 100, and other digital components, is 3.3V while thevoltage required to activate the gates of FETs 200 and 204 is 10V.

Accordingly, controller 20 employs a charge pump circuit 208 to powerthe gates of each switch. Charge pump circuit 208 receives a PWM voltage212 from processor 100 and, depending upon the duty cycle of the PWMvoltage 212, provides a voltage between 0 and an appropriately selectedmaximum voltage to the gates of switch 144. As will be apparent to thoseof skill in the art, PWM voltage 212 corresponds to the W_(1ON) logicsignal used to activate switch 144 via signal bus 122, with a zero dutycycle corresponding to switch 144 being deactivated and an appropriateselected duty cycle (e.g.—80%) corresponding to an appropriate voltageto activate switch 144.

While charge pump circuit 208, and the corresponding charge pumpcircuits associated with each other switch in relay assembly 140,provide the necessary control of the gate voltages of the correspondingFETs, they suffer from a disadvantage in that, when it is desired todeactivate switch 144, the capacitors employed as part of charge pumpcircuit 208 and the capacitance of PCB traces connecting them and othercomponents of charge pump circuit 208 prevent the output of charge pumpcircuit 208 from moving to a zero voltage fast enough to move theconnected FETs through their linear region of operation as quickly as isdesired.

Accordingly, charge pump circuit 208 is further equipped with a rapidshut down circuit 216 which operates such that when the output voltageof charge pump 208 falls below a threshold value, slightly below thenormal “on” voltage applied to activated gates of FETs 200 and 204,rapid shut down circuit will function to connect the positive output ofcharge pump circuit 208 to the negative output, thus quickly dissipatingany remaining charge in charge pump circuit 208 and the respective FETgates and bringing the voltage output to zero and thus rapidly movingFETs 200 and 204 through their linear operating region to their offcondition.

Each switch in relay assembly 140 comprising a FET based switch issimilarly equipped with an equivalent charge pump circuit and rapid shutdown circuit such that processor 100 can shut down a respective switchin relay assembly 140, when one or more of the predefined thresholdsdiscussed above is exceeded and that shut down will occur sufficientlyquickly so as to prevent damage to the respective FETs.

As should now be apparent, the present invention provides a novelcontrol device, such as a smart thermostat, which employs solid staterelays as switches to activate and deactivate systems controlled by thedevice, such as HVAC fans, compressors, etc. Current flows through thesolid state relays to at least some of the controlled systems aremonitored to determine the bulk heat produced in the solid state relaysand their associated circuitry and printed circuit board traces and thisdetermined amount of bulk heat is used, along with other determined bulkheating values, to compensate the reading provided by temperaturesensors within the control device which have been affected by the bulkheat.

Further, by measuring the current flow to each power bus in controller20, potentially damaging over current conditions can be distinguishedfrom permissible transient over-current conditions and the controldevice can deactivate any solid state relays which would be damagedwhile allowing solid state relays which are experiencing allowabletransients to remain operating. In the case of a severe over currentcondition, a current monitoring device can issue a fault signal,triggering an interrupt condition which will cause a processor in thecontroller to shut down the affected solid state relays very quickly.

The above-described embodiments of the invention are intended to beexamples of the present invention and alterations and modifications maybe effected thereto, by those of skill in the art, without departingfrom the scope of the invention which is defined solely by the claimsappended hereto.

We claim:
 1. A method of protecting at least one solid state relayconnected between a load and a power bus within a control device frompotentially damaging over current conditions, comprising the steps of:(a) determining a set of five safety thresholds, each thresholdcomprising a maximum current flow level and a maximum permitted timetherefore, the thresholds being defined according to a safe powerdissipation capacity of the at least one solid state relay, the fivesafety thresholds comprising: (1) if a maximum measured bus current fora power bus exceeds 12.5 amps, then the respective power bus switchesare deactivated; (2) if a maximum measured bus current for a power busis less than 12.5 amps but greater than 12 amps for one full AC cycle,then the respective power bus switches are deactivated; (3) if maximummeasured power bus current for a power bus is less than 12 amps butgreater than 11 amps for more than two full AC cycles, then therespective power bus switches are deactivated; (4) if maximum measuredpower bus current for a power bus is less than 11 amps but more than 10amps for more than four full AC cycles, then the respective power busswitches are deactivated; and (5) if maximum measured power bus currentfor a power bus is less than 10 amps but greater than 3.5 amps for morethan seven complete AC cycles, then the respective power bus switchesare deactivated; Wherein the maximum measured power bus current for apower bus between 0 amps and 3.5 amps is considered a nominal operatingcondition; (b) measuring a current flow through the power bus; (c)comparing that measured current flow to each of the five safetythresholds; (d) determining if the measured current flow exceeds themaximum current flow level of at least one of the five safetythresholds; (e) if the measured current flow exceeds the maximum currentflow level of the at least one of the five safety thresholds,determining if the current flow has occurred for a period exceeding themaximum permitted time defined for that threshold and shutting down atleast one of a plurality of power bus switches if the maximum permittedtime threshold has been exceeded; (f) repeating step (e) for each of thefive safety thresholds; and (g) repeating steps (b) through (f).
 2. Themethod of claim 1, wherein each of the three safety thresholds is basedon a maximum power level that a FET of an associated at least one solidstate relay can dissipate in a given time without being damaged.
 3. Themethod of claim 1, wherein steps (b) through (f) are repeated aboutevery 1 ms.
 4. The method of claim 1, wherein the maximum permitted timedefined for at least one safety threshold of the five safety thresholdsis defined in terms of a predefined number of AC cycles.
 5. The methodof claim 4, wherein: the five safety thresholds includes a lower safetythreshold and a higher safety threshold; the maximum permitted timedefined for the lower safety threshold and the higher safety thresholdare each defined in terms of respective predefined numbers of AC cycles;and instances in which a given maximum measured power bus currentexceeds the lower safety threshold are counted toward the higher safetythreshold.
 6. The method of claim 1, wherein shutting down each of theat least one solid state relays comprises receiving a fault logic signaland treating the fault logic signal as an interrupt.
 7. The method ofclaim 1, wherein the at least one solid state relay is part of anelectrical subsystem which is part of an HVAC subsystem.
 8. The methodof claim 1, wherein the at least one solid state relay comprises a pairof FETs; a charge pump to deliver a control voltage to gates of the FETSfrom a processor; and a fast shutdown circuit, operable to connect anoutput of the charge pump to ground when the fast shutdown circuitdetects that the control voltage is less than a predefined value.
 9. Amethod of protecting at least one solid state relay connected between aload and a power bus within a control device from potentially damagingover current conditions, comprising the steps of: (a) determining a setof five safety thresholds, each threshold comprising a maximum currentflow level and a maximum permitted time therefore, the thresholds beingdefined according to a safe power dissipation capacity of the at leastone solid state relay, the five safety thresholds comprising: (1) if amaximum measured bus current for a power bus exceeds 12.5 amps, then therespective power bus switches are deactivated; (2) if maximum measuredpower bus current for a power bus is less than 12.5 amps but greaterthan 12 amps for one full AC cycle, then the respective power busswitches are deactivated; (3) if maximum measured power bus current fora power bus is less than 12 amps but greater than 11 amps for more thantwo full AC cycles, then the respective power bus switches aredeactivated; (4) if maximum measured power bus current for a power busis less than 11 amps but more than 10 amps for more than four full ACcycles, then the respective power bus switches are deactivated; and (5)if maximum measured power bus current for a power bus is less than 10amps but greater than 3.5 amps for more than seven complete AC cycles,then the respective power bus switches are deactivated; wherein themaximum measured power bus current for a power bus between 0 amps and3.5 amps is considered a nominal operating condition; (b) measuring acurrent flow through the power bus; (c) comparing that measured currentflow to each of the five safety thresholds; (d) determining if themeasured current flow exceeds the maximum current flow level of at leastone of the five safety thresholds; (e) if the measured current flowexceeds the maximum current flow level of the at least one of the fivesafety thresholds, determining if the current flow has occurred for aperiod exceeding the maximum permitted time defined for that thresholdand shutting down at least one of a plurality of power bus switches ifthe maximum permitted time threshold has been exceeded; (f) repeatingstep (e) for each of the five safety thresholds; and (g) repeating steps(b) through (f).
 10. The method of claim 9, wherein each of the threesafety thresholds is based on a maximum power level that a FET of anassociated at least one solid state relay can dissipate in a given timewithout being damaged.
 11. The method of claim 9, wherein steps (b)through (f) are repeated about every 1 ms.
 12. The method of claim 9,wherein the maximum permitted time defined for at least one safetythreshold of the five safety thresholds is defined in terms of apredefined number of AC cycles.
 13. The method of claim 12, wherein: thefive safety thresholds includes a lower safety threshold and a highersafety threshold; the maximum permitted time defined for the lowersafety threshold and the higher safety threshold are each defined interms of respective predefined numbers of AC cycles; and instances inwhich a given maximum measured power bus current exceeds the lowersafety threshold are counted toward the higher safety threshold.
 14. Themethod of claim 9, wherein shutting down each of the at least one solidstate relays comprises receiving a fault logic signal and treating thefault logic signal as an interrupt.
 15. The method of claim 9, whereinthe at least one solid state relay is part of an electrical subsystemwhich is part of an HVAC subsystem.
 16. The method of claim 9, whereinthe at least one solid state relay comprises a pair of FETs; a chargepump to deliver a control voltage to gates of the FETS from a processor;and a fast shutdown circuit, operable to connect an output of the chargepump to ground when the fast shutdown circuit detects that the controlvoltage is less than a predefined value.